Within recent times, a variety of uses have arisen requiring the radiation of electromagnetic wave energy that is not itself of sinusoidal waveform and which does not employ a sinusoidal carrier. In the context of this discussion the term "sinusoidal" includes waveforms that are approximately sinusoidal such as those encountered in frequency modulation or amplitude modulation of a sinusoidal carrier. Those recently arisen uses are primarily in radar and to a lesser extent in specialized forms of radio communications usually referred to as "spread spectrum" or "frequency sharing" systems. The uses in radar include all-weather line-of-sight radars, over-the-horizon radars, and geophysical survey systems of the kind disclosed in U.S. Pat. No. 3,806,795. Some of those radar uses and other uses are discussed in the book titled "Nonsinusoidal Waves For Radar And Radio Communications" by Henning F. Harmuth, Academic Press, New York, 1981.
It is generally agreed that the most troublesome area in radio systems using nonsinusoidal waves is in providing suitable antennas for those systems, particularly the antenna used for radiation. Many types of so-called frequency independent antennas are known such as the log-spiral antenna, the horn antenna, the exponential surface antenna, etc. Such "prior art" antennas are discussed in the book "Frequency Independent Antennas" by Victor H. Rumsey, Academic Press, New York, 1966. Those antennas usually permit radiation of sinusoidal waves within a wide frequency range whereas the resonant type of antenna only permits radiation of sinusoidal waves within a relatively narrow band. However, the "prior art" frequency independent antennas usually cause significant distortions where a nonsinusoidal wave or wave energy with large relative bandwidth is radiated. Moreover, most of those "prior art" frequency independent antennas are of large physical size.
The term "relative bandwidth" is fundamental to a discussion of the transmission of nonsinusoidal waves. Relative bandwidth in conventional radio transmission means the quotient .DELTA.f/f.sub.c where .DELTA.f is the frequency bandwidth and f.sub.c is the carrier frequency of a radio signal. Nonsinusoidal electromagnetic waves, however, do not have a carrier frequency f.sub.c. Therefore, the more general definition ##EQU1## is used for the relative bandwidth, where f.sub.H and f.sub.L stand respectively for the highest and lowest frequency of interest. For a pure sinusoidal wave f.sub.H =f.sub.L and consequently the relative bandwidth is zero. The conventional sinusoidal signals used in radio, TV, radar, radio navigation, etc., typically have a relative bandwidth of 0.01 or less. The largest possible value of .eta. is 1 and applies, for example, to a rectangular pulse occupying the frequency band from zero to infinity.
Most "prior art" frequency independent antennas are useful for small relative bandwidths only, that is, for relative bandwidths of about 0.01 or less. The antenna of this invention, in contrast, can radiate and receive electromagnetic signals with a relative bandwidth .eta. of close to 1. Moreover, the antenna of this invention, when used for transmission, can be constructed of small size by trading off an increase in current for smaller size.